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Japan Raises Danger Level at Power Plant

By raising the level of the crisis at the Fukushima nuclear power station to 5 on a scale of 1 to 7, Japan’s nuclear safety agency on Friday gave it a ranking equal to that of the Three Mile Island accident of 1979. Only two events rank higher, including the 1986 Chernobyl disaster, the only accident to be rated a 7.

Yet the consensus among nuclear safety experts outside Japan is that the situation there is already worse than Three Mile Island, where a partial fuel meltdown at one reactor was contained with a relatively small release of radioactivity.

The International Atomic Energy Agency, which developed the ratings, says it is not meant to be used to compare events in different countries and at different times. Still, the Fukushima event has involved a significant release of radiation, along with damage in several reactors and spent-fuel storage pools. How could it be given the same rating as Three Mile Island?

The answer lies in the nature of the ranking system and in who is doing the ranking.

Unlike scales for events like hurricanes, which are based on measurements of a single factor like wind speed, the nuclear event scale is based on many criteria — some having to do with exposure of the public to radiation, some with the condition of the reactor fuel and still others with whether safety provisions fail. The formula is so complex that the agency publishes a 218-page “user’s manual.”

Nor are the criteria as precise as those used with some other scales. One measure of a nuclear event rated Level 5, for example, is the melting of more than the equivalent of “a few percent” of reactor fuel, the manual says. With spent-fuel storage pools, a Level 3 event involves “substantial” uncovering of the fuel rods. Those criteria are a far cry from ranking a hurricane as a Category 3 because it has sustained winds of 111 to 130 miles an hour.

Moreover, the rating is calculated by the agency where the event is occurring. This means that a government can use the number to play down a crisis if it wishes. The French nuclear agency, for instance, has said that the events in Fukushima are at Level 6. But Japan’s rating is the only one that counts.

The scale is meant to be logarithmic, meaning each level is considered 10 times as severe as the one below it. So it could be that the Japanese also believe the situation at their power plant is worse than Three Mile Island — just not 10 times worse, not yet.

A version of this article appears in print on March 19, 2011, on Page A12 of the New York edition with the headline: Japan Raises Danger Level At Power Plant. Order Reprints|Today's Paper|Subscribe